Q: We have been trying to convince our local public works officials to specify segmental retaining wall (SRW) blocks on an upcoming project. We are located in the northern part of the United States. And the county's engineering staff is concerned that the SRW blocks will not be durable given our area's numerous freeze-thaw cycles and the heavy use of deicers on the roads and sidewalks.
While our SRW products have a good service life on many area projects, we have agreed to review our mix designs in search of even a better unit. Should we consider adding an air entrainment admixture to help prevent damage from frost or deicing salt scaling?
A: Although research and service-life assessments of existing structures have shown that using air entrainment in ordinary concrete can improve frost resistance and deicing scaling resistance, there is little agreement on how successful it works in dry concrete.
First, there's the question of whether the admixtures can even cause entrainment. Some studies have shown that air entrainment is extremely difficult, regardless of the admixture dosage rate. But other studies have reported some entrainment dependent on specific conditions of admixture type, mix design, plant mixer, and batching sequence.
This opinion was reported in the Federal Highway Administration's (FHWA) newly released “Durability of Segmental Retaining Wall Blocks: Final Report.” FHWA funded a three-year study regarding the freeze-thaw durability of SRW units. Researchers from The University of Texas at Austin, Cornell University, and Texas A&M University conducted the research.
The study was initiated based on concerns expressed by transportation officials on the durability of SRW blocks used in northern climates. These systems are popular for a variety of reasons, including lower construction costs, versatility, aesthetic appearance, ease of installation, and the increasing availability of a number of proprietary designs.
“Despite these inherent advantages, there have been some reported problems with durability of segmental retaining wall blocks in cold climates,” according to the report.
In recent years, the Minnesota and Wisconsin DOTs have conducted extensive field surveys to assess the level of damage to SRWs. In Minnesota, 104 walls was each assigned a distress rating between 0 and 5, with 0 indicating the worst distress, and 5 indicating no visible distress.
While only 7% of the walls examined were classified as poor to very poor, more than 50% exhibited some type of freeze-thaw damage. Walls near parking lots or close to the road exhibited the most damage, due to increased amounts of snow accumulation and water runoff. This allows for greater saturation and exposure to deicing salts.
The walls exposed to fertilizer also exhibited more damage due to phosphates as the fertilizers behaved similar to deicing salts.
The project explored unit deterioration mechanisms in the field, identified variables that contribute to durability, and provided conclusions and recommendations for future work.
“It has been confirmed that the vast majority of segmental retaining wall blocks have performed well and continue to perform well, even in cold climates and when exposed to deicing salts,” Gary Henderson, director of the Office of Infrastructure for FHWA, wrote in the report's introduction.
The published report is available at www.tfhrc.gov/structur/pubs/07021/index.htm.